Mem. S.A.It. Vol. 84, 960
SAIt 2013c Memoriedella
Parallax measurements of six brown dwarfs ?
E. Manjavacas1, B. Goldman1, S. Reffert2, and T. Henning1
1 Max Planck Institut f¨ur Astronomie. K¨onigstuhl, 17. D-69117 Heidelberg, Germany e-mail: manjavacas@mpia.de
2 Landessternwarte K¨onigstuhl 12, D-69117 Heidelberg, Germany
Abstract. Accurate parallax measurements allow us to determine physical properties of brown dwarfs, and help us to constrain evolutionary and atmospheric models and reveal unresolved binaries. We measured absolute trigonometric parallaxes and proper motions of six cool brown dwarfs using background galaxies to establish an absolute reference frame.
The brown dwarfs in our sample have spectral types between T2.5 and T7.5. The observa- tions were taken in the J-band with the Omega2000 camera at the 3.5 m telescope at CAHA during a time period of 27 months. We obtained absolute parallaxes for our 6 brown dwarfs with a precision between 3 and 6 mas. We compared our results with the study by Dupuy et al. (2012) and with the evolutionary models of Allard et al. (2012). For four of the six tar- gets we found a good agreement in luminosity among objects of similar spectral types. The object 2MASS J11061197+2754225 is more than 1 mag overluminous in all bands pointing to binarity or higher order multiplicity.
Key words.stars: brown dwarfs – late type – infrared: astrometry – parallaxes – proper motions – stars: distances
1. Introduction
Since the discovery of the first brown dwarfs (BDs), Teide 1 by Rebolo et al. (1995) and Gliese229B by Nakajima et al. (1995), more than 1000 L and T type brown dwarfs have been discovered1.
BDs cool and dim during their evolution.
During their life, they change their spectral type because the effective temperature (Teff) decreases, with the exception of BDs in the L/T
Send offprint requests to: E. Manjavacas
? Based on observations taken with Omega-2000 at the 3.5 m telescope at the Centro Astron´omico Hispano Alem´an (CAHA) at Calar Alto, operated by the Max Planck Institut f¨ur Astronomie and the Instituto de Astrof´ısica de Andaluc´ıa (CSIC).
1 www.dwarfarchives.org
transition in which the Teffis roughly constant.
For a given BD with a given temperature, the interval of masses and ages that the BD could have is very wide, so that the age and the mass are degenerate. The Teffis the physical parame- ter that drives the major changes in the observ- able photometric and spectroscopic features of brown dwarfs. However, among the known brown dwarfs emerge outliers which show that secondary parameters are also responsible for brown dwarf properties, such as gravity, metal- licity, and cloud variability (Burrows et al.
2006; Burgasser et al. 2006; Liu et al. 2007).
Since the first parallax programs for brown dwarfs began with Dahn et al. (2002), Vrba et al. (2004), and Tinney et al. (2003), the re- lationship between the color and magnitude of BDs has been studied (Burgasser et al.
Manjavacas et al.: Parallax measurements of six brown dwarfs 961 Table 1. Summary of the results.
Object µα µδ πrel πabs d (pc) dphot χ2/Ndof
(mas yr−1) (mas yr−1) (mas) (mas) (pc) (pc)
2M J1106+2754 −311 ± 4 −438 ± 5 46 ± 3 48 ± 3 20.6+1.0−1.2 12.5 ± 1.4 21.8/17 ULAS J1302+1308 −445 ± 6 5 ± 7 67 ± 5 65 ± 5 15.4+1.1−1.4 16.1 ± 2.3 25.4/19 ULAS J1417+1330 −121 ± 4 50 ± 3 32 ± 3 33 ± 3 30.3+2.5−3.8 23.8 ± 5.1 23.0/17 2M J2254+3123 67 ± 3 187 ± 7 71 ± 2 72 ± 3 13.9+0.5−0.6 14.7 ± 1.9 23.4/17 ULAS J2320+1448 410 ± 4 121 ± 3 47 ± 3 47 ± 4 21.1+1.6−2.2 20.7 ± 3.6 22.6/19 ULAS J2321+1354 76 ± 4 −576 ± 6 83 ± 3 84 ± 4 11.8+0.5−0.6 10.8 ± 0.7 22.2/19
2008; Schilbach et al. 2009; Marocco et al.
2010; Faherty et al. 2012; Dupuy et al. 2012, among others). One of the most significant re- sults of these studies is the large dispersion in luminosity for objects with similar spectral types (Faherty et al. 2012), which shows the importance of other factors such as gravity, metallicity, sedimentation and binarity (Tsuji et al. 1996; Burrows et al. 2006; Saumon &
Marley 2008). Increasing the number of cool brown dwarfs with accurate distance measure- ments allow us to understand the variation in the color-magnitude and H-R diagrams, as we can determine the luminosity more accurately.
Also the J-band bump in the color-magnitude diagram, a brightening observed in the J band for brown dwarfs with spectral types between T1 and T5, is not well understood (Burgasser et al. 2002; Tinney et al. 2003; Vrba et al. 2004).
There are still few objects with parallaxes in the L/T transition, which prevent the progress of understanding brown dwarf evolution.
2. Data
We selected six T brown dwarfs with spec- trophotometric distances smaller than 25 pc and brighter than 18 mag in J-band. Images were taken with Omega-2000, at the 3.5 m tele- scope at CAHA. The field of view is 15.4 × 15.40, with a resolution of 0.0045 pixel−1. The baseline of our observations is 21 months. In the case of ULAS J232035.28+144829.8, we used also one observation in the methane filter from 2009; therefore the baseline is 3.25 years.
3. Analysis
We performed astrometry using the soft- ware SExtractor (Bertin & Arnouts 1996) and SCAMP (Bertin et al. 2006). We estimated the differential chromatic refraction effect of the targets as explained in Monet et al. (2013) and Stone (1996). This effect was negligi- ble in comparison with other sources of error (1 mas/epoch). We calculated the relative par- allaxes of our targets (using field stars as ref- erences), fitting positions of the targets in each epoch, proper motions and parallaxes. In order to determine the absolute parallaxes, we calcu- lated the average parallax of the extragalactic sources in our fields. Then, the absolute paral- lax is: πabsolute= πrelative− πextragal..
4. Discussion of the results
Our estimation of the luminosity agrees with the luminosities of objects with similar spec- tral types for four of our six targets. The ob- ject ULAS J141756.22+133045.8 is slightly overluminous (around 0.5 mag) in the WISE color-magnitude plot. The object 2MASS J11061197+2754225 is more than 1 mag over- luminous in all the bands, pointing to binarity or even higher multiplicity. In order to con- firm this result, high resolution imaging and high resolution spectroscopy would be needed.
To summarize, we provided parallaxes of some targets for the first time, with a precision up to 4 mas yr−1in proper motions and 3 mas in par- allaxes.
Acknowledgements. We acknowledge J. A.
Caballero, M. R. Zapatero Osorio, V. J. S. B´ejar,
962 Manjavacas et al.: Parallax measurements of six brown dwarfs
Fig. 1.Color-magnitude diagram in the MKO system showing the brown dwarf sample from Dupuy et al.
(2012), our targets and the BT Settl models (Allard et al. 2012).
Wolfang Brandner, and Amelia Bayo for their help, their corrections, constructive criticism and their useful advice.
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